EVEN as science fiction, the idea that we can learn as we sleep has a controversial history. In Aldous Huxley’s dystopian novel Brave New World, recorded voices whisper class prejudices into the ears of sleeping children, conditioning them for their future roles in society. Despite the evil ends that Huxley imagined, the appeal of getting something for nothing was irresistible to readers and, following the book’s publication in 1932, there was a surge in interest in sleep learning. But was there any fact in the fiction?

In 1951, two researchers at George Washington University in Washington DC decided to find out. They recruited 30 volunteers and set up tape recorders and speakers in their bedrooms. For half an hour one night, as the volunteers slept, the recorders played either music or Chinese words and their English equivalents. Next morning, those who had heard the vocabulary performed better on a Chinese language test. “Learning can occur during sleep,” the scientists concluded, grandly. Others soon agreed. A second team claimed to have taught Morse code to a group of sleeping naval students. A third found that children stopped biting their nails simply by listening to the sentence, “My fingernails taste terribly bitter”, six times a night for 54 nights.

But it wasn’t long before these findings were challenged. The volunteers hadn’t been monitored, so there was no evidence they were truly asleep as the audio played. So in 1955, researchers did several more studies, this time measuring brain activity using electroencephalography (EEG) to make sure the recordings were played only during sleep. No learning was detected. The original claims were discarded, and …